Radio frequency window cooling structure and transmission devices using same



MISS ION 2 Sheets-Sheet 1 March 7, 1967 B155 RADIO FREQUENCY WINDOWCOOLING STRUCTURE AND TRANS DEVICES USING SAME Filed July 16, 1963INVENTOR RICHARD H. L. BIBB ATTORNEY r h 7, 1907 I R. H. L. BIBB3,308,332

RADIO EREQUENCY WINDOW COOLING STRUCTURE AND TRANSMISSION DEVICES USINGSAME Filed July 10, 1003 2 Sheets-Sheet 2 2h? W/l l- FIG .5 2| FIG. 6

A- 00000000000 0 0 0000 0000 0000020 6 26 OFF "R996 000 003/ 2 00 0 g 2600 00 F "Q 0 a 0& Q50 00 g 000 28 00 320 as 22 32 )3 OF A we e fi\26L/3e gr )0 8 ?8 00 v0 00 00 (6 9 7 000 8555560000550006000060000650000 INVENTOR. RICHARD H. L. BIBB ATTORNEY UnitedStates Patent Ofifice 3,308,332 Patented Mar. 7, 1967 3,308,332 RADIOFREQUENY WINDQW CGOLKNG STRUC- 'SlEIQI EE AND TRANSMISSEON DEVICES USINGE Richard H. L. Bibi), Pain Alto, Calif., assignor to Varian Associates,Palo Alto, Calif., a corporation of California 7 Filed July 16, 1963,Ser. N 295,481 12 Claims. (Cl. 315-) The present invention relates ingeneral to high frequency electron tube apparatus and more particularlyto an electromagnetic wave permeable window cooling structure.

As in all vacuum tubes, super power, microwave frequency tubes mustprovide therewithin a high vacuum to permit creation of an electron beamfor the generation and amplification of microwave signals. High powermicrowave energy is extracted from these tubes via an output waveguideprovided with an electromagnetic wave permeable window, typically ofalumina ceramic, which is vacuum sealed within the output waveguide tomaintain the vacuum within the tube and to permit passage ofelectromagnetic waves therethrough without substantial energy loss.

As the state of the art of microwave tubes has advanced, one of thelimiting factors on the amount of microwave power that can be producedwith a single tube is the wave permeable window in the output waveguide.The window must be able to transmit the high power microwave energywithout cracking or rupturing while at the same time maintaining thehigh vacuum within the tube. For example, the output waveguide on atypical L band frequency microwave tube capable of producing 100kilowatts of average power is approximately 7" diameter and the vacuumwithin such a tube is maintained at a level on the order of 1 10- mm. ofHg. Such high output powers necessarily dissipate, for example, severalhundreds of watts of average power dissipated in the window as heat. Theresultant temperature rise produces thermal stress which will rupturethe window or supporting structure unless additional cooling is added.

Water and air-cooling schemes attempted heretofore for super power tubewindows have caused uneven cooling of the window. Uneven coolingproduces stresses across the window which have caused windows to crackand rupture with resultant loss of vacuum of the electron tube.

Previous watercooling arrangements for cooling the periphery of thewindow have not solved the problem of temperature gradients in thecenter of the window. Furthermore, in the previous water-cooling schemesin which water or other coolants are passed through conduits in thewindow itself, the window is extremely dithcult to fabricate, and thewater passing through the window presents high loss to theelectromagnetic wave passing therethrough.

Experiments with air streams on high power tube output windows haveheretofore been relatively unsuccessful because of the lack of abilityto get the majority of the air stream close enough to the window foreffective cooling and because of uneven cooling of the window due to airvortices created in the waveguide at the window region.

In accordance with the present invention, to be described in greaterdetail below, the electromagnetic wave permeable window is positionedwithin a waveguide, and a plurality of closely grouped gas directiveapertures are provided in one side wall of the waveguide with theapertures spaced from the window along the longitudinal axis of thewaveguide and with the axes of the apertures directed substantially atthe window. Another group of gas apertures is provided in the waveguidewall opposite the one wall. It has been discovered that with the axes ofthe apertures inclined at an angle of between 30 and 60, and preferablysubstantially 45, with respect to the longitudinal axis of thewaveguide, substantially more uniform and greater cooling over theentire window is achieved as compared to cooling obtained by otherarrangements of apertures and coolant flow.

The principal object of the present invention is to provide an improvedapparatus for cooling electromagnetic wave permeable windows, especiallyuseful for super power electron discharge devices.

One feature of the present invention is the provision of anelectromagnetic wave permeable window Within a waveguide and a pluralityof closely grouped gas directive apertures in one wall of the waveguidespaced longitudinally along the waveguide from the window and aplurality of closely spaced gas apertures in the opposite waveguidewall, the axes of the apertures in the one wall of the waveguide beingdirected at the window for directing a gas across the window and out theopposite side of the waveguide.

Another feature of the present invention is the provision of a coolingstructure according to the preceding feature wherein the axes of theaperture in the first waveguide wall are inclined at an angle of between30 and 60 and preferably substantially 45 with respect to thelongitudinal axis of the waveguide.

Another feature of the present invention is the provision of thestructure of the preceding featurehaving the wave permeable windowmounted substantially transversely within a rectangular waveguide andhaving the apertures for directing gas across the window located in thenarrow walls of the waveguide whereby waveguide arcs, if any, are blownaway from the window.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

FIG. 1 is a longitudinal foreshortened view, partly in section, of ahigh frequency electron discharge device employing features of thepresent invention;

FIG. 2 is a perspective view of the output window and associatedwaveguide, partially broken away, illustrated in FIG. 1 with one airduct removed;

FIG. 3 is an end view of the structure shown in 'FIG. 2 taken along line3-3 looking in the direction of the arrows;

FIG. 4 is a cross-sectional foreshortened view of the structure shown inFIG. 2 taken along line 44 looking in the direction of the arrows;

FIG. 5 is a view of the side of the structure shown in FIG. 4 takenalong line 55 looking in the direction of the arrows; and

FIG. 6 is a view similar to FIG. 5 showing an alternative arrangement ofthe plurality of closely grouped apertures in the waveguide wall.

Referring now to FIG. 1, there is shown a high frequency electrondischarge tube apparatus utilizing features of the present invention.More particularly, the tube comprises an evacuated tubular envelope 1evacuated to a suitable low pressure as of, for example, 1X10millimeters of mercury via the intermediary of an appendage ion pump 2in gas communication with the interior of the tube envelope 1 via asuitable tubulation 3.

An electron gun assembly 4 is disposed at one end of the tube envelope 1and serves to form and project a beam of electrons over a predeterminedpath directed axially and longitudinally of the tube envelope 1. A beamcollecting structure 5 is disposed at the terminating end of theelongated electron beam path for collecting the electron beam. A coolantsuch as water is supplied to the beam collecting structure via fluidfittings 6 and circulates through suitable ducts, not shown, in thecollector structure 5.

A plurality of re-entrant cavity resonators 7 and 8 are arranged alongthe beam path in axially spaced re'ation for electromagnetic interactionwith the electron beam passable therethrough. Input wave energy to beamplified is supplied to the input resonator 7 via the intermediary ofan input loop and a coaxial line 11. Amplified output Wave energy isextracted in the conventional manner from the beam via output resonator8 and propagated to a suitable load, not shown, via the intermediary ofan output iris 12 and output waveguide 13 sealed in a suitable vacuumtight manner via the intermediary of a wave permeable vacuum tightwindow 14 mounted substantially transversely of the waveguide 13 by awindow frame 15.

An electric solenoid (not shown) coaxially surrounds the elongatedvacuum envelope 1 and provides an axially directed beam focusingmagnetic field for confining the beam to its predetermined beam path.

Movable tuning structures (not shown) are provided within the cavityresonators 7 and 8 for tuning of the tube over the operating frequencyrange.

In operation, input signals are applied to the input resonator 7 viacoaxial line 11. The signals are amplified in successive resonators andthe amplified outward signals are derived from the tube 1 via the outputwaveguide 13. The window 14 is cooled during operation of the tube bymeans of an air stream directed into Waveguide 13 across the outsidesurface of window 14 and then again out of the waveguide 13 as set forthin greater detail below.

As part of the output waveguide 13 a rectangular waveguide 17 outsidethe vacuum envelope is provided including a pair of opposed broad walls18 and 19 and a pair of opposed narrow walls 21 and 22. The rectangularwaveguide 17 is provided on one end with a flange 23 mating with awindow flange 24 surrounding the window 14 and window frame 15. Theother end of the rectangular waveguide 17 is provided with a flange 25for coupling the tube to the load (not shown).

A plurality of closely grouped gas directive input apertures 26 areprovided in the first narrow waveguide wall 21 for directing a coolinggas such as, for example, air onto the window 14. On the outside of thenarrow wall 21 is provided a duct 27 constructed and arranged to receivean air pipe 28 from an air supply such as an air blower (not shown).Also a plurality of closely grouped outlet apertures 31 are provided inthe opposite narrow waveguide wall 22 for permitting passage of thecooling air stream out of the waveguide 17. Although not necessary forproper operation of the invention, a duct 32 and an air pipe 33, similarto duct 27 and air pipe 28, respectively, are provided for conveniencearound the apertures 31.

The air stream from theair pipe 28 passes through the ducts 27 and isdirected by the inlet apertures 26 across the window and out through theoutlet apertures 31. The air flow path is illustrated in FIG. 4. Notehow the coolant flow path decreases in thickness as it flows across thewindow member due to a venturi eifect, i.e., a reduction in pressure atthe center of the window. This pulling in of the coolant flow greatlyfacilitates cooling of the window as a greater volume of coolant iscaused to flow in closer contact with the window member than otherwiseobtained with coolant flows that do not exhibit the venturi effect.

As set forth in greater detail below, a certain inclination of the axesof the apertures 26 with respect to the longitudinal axis of thewaveguide 17 as well as a spacing of the pattern of apertures from theplane of the window are preferred for evenly cooling the window.

The inlet apertures 26 serve to give directivity to the air streamdirected therethrough and also serve as waveguides beyond cut-olf forpreventing R.F. leakage without producing large head losses to the airstream and without introducing a mismatch or encouraging spurious modesin the waveguide. It has been discovered that an open duct does notprovide sufficient directivity to a cooling air stream to cool thewindow 14 properly and the use of baffles to accomplish this woulddeleteriously effect the RF. characteristics of the waveguide.

It is preferred to have at least as many output apertures 31 as inputapertures 26 and preferably, as many ou t ape tu e 31 as oossibie areprovided so that the resistance presented to the air stream leaving thewindow is as low as possible.

While the apertures 26 and 31 can be in the broad walls 18 and 19 of thewaveguide 17, it is preferable that they be located in the narrow sidewalls 21 and 22 since high power arcs might knock out the sections ofwaveguide between adjacent apertures. These arcs, which may be createdwithin waveguide 17, take the shortest path between waveguide walls,between the broad walls 18 and 1%.

It has been discovered that the group of gas directive apertures 26should be spaced longitudinally of the waveguide 17 away from the planeof the window 14. If the group of gas directive apertures 26 is tooclose to the plane of the window, the air stream will follow a paththat, after reaching the window, flows away from the window rather thanparallel to it. On the other hand, a group of apertures spaced from theplane of the window directs the air stream to and then along the window.However, as the group of apertures 26 is placed further and further downthe waveguide 17, the air stream will fail to adequately cool theclosest edge of the window. Therefore, the group of apertures ispreferably spaced longitudinally of the waveguide a distance which isdependent upon the waveguide and window dimensions in each particularinstance. Also, the best pattern of the apertures 26 will depend uponthe waveguide and window dimensions and therefore will usually bedetermined by trial and error. This pattern can either be circular asshown in FIG. 5, square as shown in FIG. 6 or of another shape dependentupon the window and waveguide dimensions.

It has been discovered that the inclination of the axes of the gasdirective aperture 26 with respect to the longitudinal axis of thewaveguide 17 is critical if efficient and even cooling of the window isto be achieved without the creation of air vortices within thewaveguide. In this regard it has been discovered that when the axes ofthese apertures 26 are inclined at angles of 30 or less or at angles of60 or more, air vortices are produced within the waveguide 17 while airvortices are not likely to be produced with angles of inclinationbetween 30 and 60. Furthermore, experiments have shown that an angle ofinclination of substantially 45 produces the best results.

In tests made using a /8" thick, 7 diameter circular window in a 6" by12" rectangular waveguide, two kilowatts of power were applied to thewindow, and tempera ture contours were plotted on the window using anair stream from a four horsepower blower directed across the window.Temperature sensitive wax was applied to the window in patterns such asshown at 41 on the window in FIG. 3. After testing a number of differentinlet aperture entrance angles, the critical entrance angle ofsubstantially 45 was established. A 6" diameter circular pattern of fivehundred and fifty seven /s" diameter inlet apertures, centered in thenarrow waveguide side wall, was moved toward and away from the plane ofthe window until an even cooling of the window was achieved.- Evencooling was obtained when the pattern center was located 5" down theguide along the axis of the guide from the plane of the window.

With these parameters and with identical inlet and outlet patternsapproximately of the air flow was within /2" of the Window with verylittle static air pressure loss in passing through the inlet apertures.Also with this construction the horsepower requirements were reduced byas much as the factor of 3 with a greatly improved cooling pattern overother cooling schemes tested. In addition with this cooling arrangementthe window had a circular constant temperature contour of 350 C., 4" indiameter, centered only A" away from the center of 5 the window.

Utilizing a fixed total aperture area, apertures with diameters of /2",/8", and were tested with both /2" and waveguide wall thickness. While/2" diameter holes in a /2" thick wall produced the highest flow rateacross the window, this combination did not produce an air stream asclose to the window as desired for best cooling. The /8" diameterapertures in a waveguide wall W thick produced the best flow rate at thewindow surface.

Cooling schemes of this generalnature have been used successfully tocool windows on existing super power microwave tubes. The parameters fortwo such schemes are as follows:

from window. Inlet aperture inclinatio Outlet aperture pattern size -tOutlet aperture pattern distance from window. Outlet apertureinclination angle Outlet aperture diameter Number of outlet aperturesWhile the invention has been described thus far as applied for use inrectangular waveguide, it is obvious that the invention is equallyapplicable to circular waveguide. Also, while the invention has beendescribed as used with an air stream for cooling the window, any gasstream could be used.

While the angle of inclination of the outlet apertures is preferably thesame as that of the inlet apertures this parameter is not as critical asthe angle of inclination of the inlet apertures for even cooling of thewindow so long as suflicient apertures are provided to offer a lowresistance to air flow leaving the window for preventing th generationof vortices.

While the invention has been described thus far with respect to theoutput window of a super power tube, it is obvious that the inventioncan be utilized for use with a window which must withstand high poweranywhere in a microwave system.

What is claimed is:

1. A microwave transmission structure for transmitting high powerelectromagnetic waves comprising:

wall means defining a waveguide for the electromag netic waves andhaving a longitudinal axis taken in the direction of power flow alongsaid waveguide, an electromagnetic wave permeable window,

means for supporting said wave permeable window within said waveguide,

said waveguide wall means provided with an array of apertures on a firstside of said waveguide for passing gas into said waveguide toward saidwindow and an array of apertures on a second side of said waveguidesubstantially opposite said first side for passing gas out of saidwaveguide, the axes of said apertures being directed toward said window,and said array of apertures having a center spaced from said windowalong the longitudinal axis of said waveguide from said window, wherebygas directed int-o said waveguide through said apertures on the firstside of said waveguide causes the gas to flow across said window and outof said waveguide through said apertures on the second side to cool saidwindow that is heated by electromagnetic wave energy dissipated therein.

2. The microwave transmission structure of claim 1 characterized furtherin that the axes of at least said apertures on said first side of saidwaveguide are inclined at an angle between 30 and, 60 with respect tothe longitudinal axis of said waveguide.

3. The microwave transmission structure of claim 1 characterized furtherin that the axes of at least said apertures on said first side of saidwaveguide are inclined at an angle of substantially 45 with respect tothe longitudinal axis of said. waveguide.

4. A microwave transmission structure for transmitting high powerelectromagnetic waves comprising:

wall means defining a waveguide for conducting electromagnetic waves,said wall means including first and second opposed side walls defining alongitudinal axis directed along the direction of power flow in saidguide,

an electromagnetic wave permeable window,

means for supporting said wave permeable window in a vacuum tight mannersubstantially transversely across said waveguide whereby said waveguideon one side of said window can be maintained under substantial vacuum,

said first side wall of said waveguide wall means on the opposite sideof said window from the vacuum being provided with an array of closelygrouped inlet gas directive apertures,

said apertures spaced longitudinally of said waveguide from said windowand the axes'of said apertures directed substantially at said window forpassing air into said waveguide toward said window, said second sidewall of said waveguide wall means on said opposite side of said windowfrom said vacuum being provided with an array of closely grouped outletgas apertures, and

means for directing gas into said waveguide through the inlet aperturesin said first side wall whereby the gas flows across said window evenlyto cool said window and out of said waveguide through the outletapertures in said second side wall of said waveguide.

5. The microwave transmission structure according to claim 4characterized further in that said axes of the apertures in at leastsaid first side wall of said waveguide are inclined at an angle between30 and 60 with respect to the longitudinal axis of said waveguide.

6. The microwave transmission structure according to claim 4characterized further in that said axes of the apertures in at leastsaid first side wall of said waveguide are inclined at an angle ofsubstantially 45 with respect to the longitudinal axis of saidwaveguide.

7. An electron tube apparatus including:

a vacuum envelope,

means for forming and projecting a beam of electrons over an elongatedpredetermined beam path within said envelope,

means for collecting the beam at the terminal end of the beam path,

wave interaction means disposed within said envelope along the beam pathintermediate said beam forming means and said beam collecting means forelectromagnetic interaction with the beam,

waveguide output means for extracting electromagnetic wave energy fromthe beam including opposed first and second side walls defining alongitudinal axis directed in the direction of power flow in saidwaveguide,

an electromagnetic wave permeable window,

means for supporting said wave permeable window substantiallytransversely within said waveguide output means in a vacuum tight mannerwhereby said window serves as a portion of said vacuum envelope,

said first side wall of said Waveguide output means on the outside ofsaid window provided with a plurality of closely grouped inlet gasdirective apertures, said apertures spaced longitudinally of said outputWaveguide means from said window and the axes of said apertures directedsubstantially at said window for passing gas into said waveguide towardsaid window,

said second side wall of said output waveguide means on said outside ofsaid window provided with a plurality of closely grouped outlet gasapertures for passing gas out of said waveguide, and.

means for directing a gas into said waveguide through said inletapertures in said first side wall of said output waveguide whereby thegas flows across said window evenly to cool said window and out of saidwaveguide output means through the outlet apertures in said second sideWall of said waveguide output means.

8. The apparatus according to claim 7 characterized further in that saidaxes of the apertures in at least said first side wall of said waveguideare inclined at an angle between 30 and 60 with respect to thelongitudinal axis of said waveguide.

9. The apparatus according to claim 7 characterized further in that saidaxes of the apertures in at least said first side wall of said Waveguideare inclined at an angle of substantially 45 10. A microwavetransmission structure for transmitting high power electromagnetic wavescomprising:

wall means defining a rectangular waveguide for electromagnetic wavesand having pairs of opposed broad and narrow side walls directedlongitudinally of said waveguide,

an electromagnetic Wave permeable window,

means for supporting said wave permeable window substantiallytransversely within said waveguide in a vacuum tight manner whereby saidwaveguide, on the inside of said window may be maintained. undersubstantial vacuum,

one of said narrow side walls on the outside of said window beingprovided with a plurality of closely grouped inlet gas directiveapertures, said apertures spaced longitudinally of said waveguide fromsaid window and the axes of said apertures directed substantially atsaid wind-ow, 1

the other narrow side wall of said waveguide be ng provided with aplurality of closely grouped out et gas apertures for passing gas out ofsaid waveguide, and

means for directing gas into said waveguide through said inlet aperturesin said one narrow side wall,

across the window evenly to cool said window and.

out through the outlet apertures in said other narrow side wall. 11. Themicrowave transmission structure according to c aim 10 characterizedfurther in that said axes of at least the inlet apertures in said onenarrow side wall are inclined at an angle between 30 to 60 with respectto the longitudinal axis of said waveguide.

12. The microwave transmission structure according to claim 10characterized further in that said, axes of at least the inlet aperturesin said one narrow side wall are inclined at an angle of substantially45 with respect to the longitudinal axis of the waveguide.

References Cited by the Examiner UNITED STATES PATENTS 1/1958 Robinson 3l3-.3'5

1. A MICROWAVE TRANSMISSION STRUCTURE FOR TRANSMITTING HIGH POWERELECTROMAGNETIC WAVES COMPRISING: WALL MEANS DEFINING A WAVEGUIDE FORTHE ELECTROMAGNETIC WAVES AND HAVING A LONGITUDINAL AXIS TAKEN IN THEDIRECTION OF POWER FLOW ALONG SAID WAVEGUIDE, AN ELECTROMAGNETIC WAVEPERMEABLE WINDOW, MEANS FOR SUPPORTING SAID WAVE PERMEABLE WINDOW WITHINSAID WAVEGUIDE, SAID WAVEGUIDE WALL MEANS PROVIDED WITH AN ARRAY OFAPERTURES ON A FIRST SIDE OF SAID WAVEGUIDE FOR PASSING GAS INTO SAIDWAVEGUIDE TOWARD SAID WINDOW AND AN ARRAY OF APERTURES ON A SECOND SIDEOF SAID WAVEGUIDE SUBSTANTIALLY OPPOSITE SAID FIRST SIDE FOR PASSING GASOUT OF SAID WAVEGUIDE, THE AXES OF SAID APERTURES BEING DIRECTED TOWARDSAID WINDOW, AND SAID ARRAY OF APERTURES HAVING A CENTER SPACED FROMSAID WINDOW ALONG THE LONGITUDINAL AXIS OF SAID